1. Field of the Invention
The present invention relates to duplexers connected to an antenna terminal in a communication apparatus, such as a mobile telephone, and, more particularly, to a duplexer including a reception filter that utilizes an elastic wave filter device including a plurality of IDTs arranged on a piezoelectric substrate.
2. Description of the Related Art
Components included in an RF stage connected to an antenna in a mobile telephone have been combined to enable miniaturization of the mobile telephone. To achieve the miniaturization, a duplexer including an antenna terminal, a transmission band-pass filter, a reception band-pass filter, a transmission terminal, a reception terminal, and an output terminal is commonly used. Furthermore, a balanced duplexer has been developed in which a balanced band-pass filter having a balanced-to-unbalanced conversion function is used as a reception band-pass filter. Such a balanced duplexer includes an antenna terminal, a transmission terminal, a first reception output terminal, and a second reception output terminal.
Japanese Unexamined Patent Application Publication No. 2003-249842 discloses a balanced duplexer illustrated in FIG. 16. In a balanced duplexer 1001, an antenna terminal 1002 is connected to a transmission filter 1003 and a reception filter 1004. The transmission filter 1003 has a ladder circuit configuration in which a plurality of surface acoustic wave resonators are connected in a ladder arrangement. The transmission filter 1003 having a ladder circuit configuration includes a transmission terminal 1005 at one end thereof that is opposite to the other end connected to the antenna terminal 1002.
On the other hand, the reception filter 1004 includes an input terminal 1006 connected to the antenna terminal 1002, a first reception output terminal 1007, and a second reception output terminal 1008. The input terminal 1006 is connected to 3-IDT longitudinally coupled resonator surface acoustic wave filter portions 1009 and 1010. The longitudinally coupled resonator surface acoustic wave filter portions 1009 and 1010 are connected to 3-IDT longitudinally coupled resonator surface acoustic wave filter portions 1011 and 1012, respectively. The longitudinally coupled resonator surface acoustic wave filter portions 1009 and 1011 are cascade connected. The longitudinally coupled resonator surface acoustic wave filter portions 1010 and 1012 are similarly cascade connected.
One end of the center IDT of the longitudinally coupled resonator surface acoustic wave filter portion 1011 and one end of the center IDT of the longitudinally coupled resonator surface acoustic wave filter portion 1012 are connected to each other and are then electrically connected to the first reception output terminal 1007. The other ends of the center IDTs of the longitudinally coupled resonator surface acoustic wave filter portions 1011 and 1012 are connected to each other and are then electrically connected to the second reception output terminal 1008.
In the reception filter 1004, the ratio of input impedance to input impedance is set to about 1:1. Furthermore, as described above, since input electric power is distributed between the longitudinally coupled resonator surface acoustic wave filter portions 1009 and 1010 in the reception filter 1004, the power handling performance can be improved.
Japanese Unexamined Patent Application Publication No. 2003-347964 discloses a 3-IDT longitudinally coupled resonator surface acoustic wave filter 1021 illustrated in FIG. 17. The longitudinally coupled resonator surface acoustic wave filter 1021 includes a first IDT 1022, a second IDT 1023, and a third IDT 1024 which are arranged along a surface acoustic wave propagation direction. The center IDT 1023 includes a first sub-IDT portion 1023a and a second sub-IDT portion 1023b which are obtained by dividing the center IDT 1023 in the surface acoustic wave propagation direction. The IDTs 1022 and 1024 are connected to an input terminal 1025. The first sub-IDT portion 1023a and the second sub-IDT portion 1023b are connected to a first output terminal 1026 and a second output terminal 1027, respectively.
The 3-IDT longitudinally coupled resonator surface acoustic wave filter 1021 can be used as a reception filter in the above-described balanced duplexer. In the longitudinally coupled resonator surface acoustic wave filter 1021, a ratio of output impedance to input impedance is set to a value greater than about one. Accordingly, if the longitudinally coupled resonator surface acoustic wave filter 1021 is used as a reception filter in the above-described balanced duplexer, impedance matching between a balanced output including first and second reception outputs, and a balanced input located at a subsequent stage of the reception filter in a mobile telephone can be easily achieved.
In the balanced duplexer 1001 disclosed in Japanese Unexamined Patent Application Publication No. 2003-249842, the power handling performance is improved in the reception filter 1004 as described above. However, if the first reception output terminal 1007 and the second reception output terminal 1008, which are included in the reception filter 1004, are connected to a balanced input included in the subsequent stage of the reception filter 1004 in a mobile telephone, impedance matching cannot be achieved since the ratio of input impedance to output impedance of the reception filter 1004 is about 1:1. As a result, a large ripple is generated in a passband.
As described above, if the longitudinally coupled resonator surface acoustic wave filter 1021 disclosed in Japanese Unexamined Patent Application Publication No. 2003-347964 is used as a reception filter in the above-described balanced duplexer, impedance matching between the reception filter and a subsequent stage can be easily achieved. However, the longitudinally coupled resonator surface acoustic wave filter 1021 does not have sufficient power handling performance. Accordingly, if the longitudinally coupled resonator surface acoustic wave filter 1021 is used in a duplexer, the longitudinally coupled resonator surface acoustic wave filter 1021 can be easily damaged by the electric power supplied thereto from a transmission filter.
A relatively large amount of electric power enters a reception filter, especially if a transmission filter is a ladder surface acoustic wave filter having a ladder circuit configuration in which a plurality of surface acoustic wave resonators are connected. Accordingly, in this case, higher power handling performance is required for the reception filter. Accordingly, a filter having insufficient power handling performance, for example, the longitudinally coupled resonator surface acoustic wave filter 1021, cannot be used as a reception filter in a duplexer including a ladder surface acoustic wave filter that functions as a transmission filter.
Examples of a surface acoustic wave filter have been described. As similar filters, elastic boundary wave filters are known. Similar to surface acoustic wave filters, elastic boundary wave filters include reflectors and an IDT, which are made of a thin metal film, on a piezoelectric substrate. For example, an elastic boundary wave filter is obtained by providing an Al filter electrode including an IDT and reflectors on the surface of a piezoelectric monocrystal substrate and providing an SiO2 thin film having a sufficient thickness on the filter electrode. The SiO2 thin film has an elastic constant or density that is different from that of the piezoelectric monocrystal. Although the principle of operation of elastic boundary wave filters and the structure of elastic boundary wave filters are similar to those of surface acoustic wave filters, the elastic boundary wave filters have a solid layer disposed on the surface of a piezoelectric substrate. The elastic boundary filters are operated by the interaction between an IDT and an elastic wave (elastic boundary wave) propagating through the boundary between the piezoelectric substrate and the solid layer. In contrast to surface acoustic wave filters that require a package having a cavity to protect the surface of a substrate, boundary acoustic wave filters have an advantage in that they do not require such a package having a cavity since a wave propagates through a boundary surface between a piezoelectric monocrystal substrate and a thin film.
Surface acoustic wave filters operate using a surface acoustic wave propagating on the surface of a piezoelectric substrate, whereas elastic boundary wave filters operate using an elastic boundary wave propagating through the boundary between a piezoelectric substrate and a solid layer. Basically, both types of filters have similar principles of operation. Furthermore, similar design methods used for these types of filters.